1. Field of the Invention
The present invention relates to a rice huller for grain, and in particular, to a rice hulling roll driving apparatus in the rice huller.
2. Description of the Related Art
In conventional rice hullers of this kind, a pulley and a driving motor attached to respective roll shafts of a pair of rice hulling rolls (main shaft roll and sub shaft roll) are connected together via an endless belt. These rice hulling rolls, that is, a high- and low-speed side rice hulling rolls, have respective constant rotational frequencies. The high-speed side rice hulling roll is more significantly worn than the low-speed rice hulling roll. A continuous rice hulling operation gradually reduces the outer diameter of the high-speed side rice hulling roll, thereby reducing the difference in peripheral speed between the high- and low-speed side rice hulling rolls. In this case, the high-speed rice hulling roll needs to be replaced with a rice hulling roll having a larger diameter than the low-speed rice hulling roll.
Thus, a rice huller configured as described below has been proposed (see Japanese Patent Application Laid-Open No. 2001-38230). Driving motors having adjustable rotational frequencies are connected to the respective roll shafts of the high- and low-speed side rice hulling rolls. Sensors are provided each of which senses the rotational frequency and outer diameter of the corresponding rice hulling roll. When the difference in peripheral speed between the rice hulling rolls which is determined by the sensors is equal to or smaller than a predetermined value, the rotational frequency of each rice hulling roll is adjusted to maintain a predetermined difference in peripheral speed. This rice huller thus eliminates the need to replace the rice hulling rolls with new ones.
With the driving apparatus in the conventional, common rice huller, the paired rice hulling rolls rotate almost in contact with each other via unhulled rice as material. This causes each rice hulling roll to exert a repulsive force. These repulsive forces act in a direction in which an endless belt, passed between the pulleys attached to the respective roll shafts in a manner such that these pulleys rotate in directions opposite to each other, is tensed. The reaction forces thus offset each other. This phenomenon will be explained with reference to FIG. 6.
A high-speed side rice hulling roll pulley 100 and a low-speed side rice hulling roll pulley 102 are driven by an endless belt 106 diagonally passed between the pulleys, as shown in FIG. 6. The rice hulling pulleys 100 and 102 then rotate at different speeds in directions opposite to each other. This causes each of paired rice hulling rolls 101 and 103 to exert a repulsive force during rice hulling. Specifically, the high-speed side rice hulling roll 101 exerts a force F1 acting in an anti-rotational direction under the effect of the low-speed side rice hulling roll 103. In contrast, the low-speed side rice hulling roll 103 exerts a force F2 acting in a rotational direction under the effect of the high-speed side rice hulling roll 101. The forces F1 and F2 act to tense the endless belt 106, passed between the paired pulleys 100 and 102, to maintain the gap between the rolls. This enables a rice hulling operation to be smoothly performed.
However, the above-mentioned Japanese Patent Application Laid-Open No. 2001-38230 connects the different driving motors to the high- and low-speed side rice hulling rolls. Thus, avoiding the repulsive forces (F1 and F2) of the rolls, which may be exerted during rice hulling, disadvantageously requires a driving force strong enough to make these repulsive forces negligible.